1. Field of the Invention
The present invention relates to an optical scanning apparatus having a light emission unit that emits plural light beams, and relates to a control method for the optical scanning apparatus.
2. Description of the Related Art
Conventionally, various techniques have been proposed to improve the accuracy of beam scanning position in an image forming apparatus of a type where a light beam (such as laser light modulated according to an image signal) is deflected and scanned by a rotary polygonal mirror across a photosensitive drum, whereby an image is formed.
In recent years, the rotation speed of the rotary polygonal mirror has been increased and in addition, multi-beam laser has become progressively used in order to increase the scanning speed to thereby improve the printing speed.
As for multi-beam image formation, a variety of methods have been proposed to improve the accuracy of scanning position, especially, for keeping a main scanning scale factor constant. As a typical example, there is a method for correcting the main scanning scale factor in realtime by using laser-light detection sensors disposed outwardly of opposite ends of the photosensitive drum or disposed at optically equivalent positions.
FIG. 12 shows the construction of a conventional exposure device mounted on a printer for detecting the laser-light scanning speed used for correction of the main scanning scale factor. The exposure device includes a laser light emission unit 101, collective lens (collimater lens) 102, rotary polygonal mirror (polygon mirror) 103, imaging lens 104, laser-light detection sensors 105, 106, and photosensitive drum 107.
The laser light emission unit 101 emits laser light 108 modulated according to image data. The laser light 108 is made into parallel beams by the collective lens 102, and deflected by the polygon mirror 103. The laser light 108 is then irradiated through the imaging lens 104 onto the photosensitive drum 107 whose surface is uniformly charged, whereby the drum 107 is scanned with the laser light in a main scanning direction. In synchronism with the scanning with the laser light 108, the photosensitive drum 107 is rotatably driven. As a result, the laser light 108 and the photosensitive drum 107 move relative to each other in a sub-scanning direction, whereby a two-dimensional electrostatic latent image is formed on the photosensitive drum 107. At that time, a timing of starting writing of each line of the electrostatic latent image in the main scanning direction is adjusted based on a detection output from the laser-light detection sensor 105.
Next, the electrostatic latent image formed on the photosensitive drum 107 is developed by adhering toner charged with opposite polarity to the electrostatic latent image, and the developed image is transferred to a recording sheet (not shown).
A method for correcting the main scanning scale factor is described in, e.g., Japanese Laid-open Patent Publication No. 2006-150696. In this method, laser-light detection sensors 105, 106 are used to measure a scanning time required for laser light 108 to pass through from the sensor 105 to the sensor 106, and the main scanning scale factor is corrected based on the measured scanning time.
FIG. 13 shows a positional relation between laser-light detection sensors and scanning loci of plural pieces of laser light. As shown in FIGS. 12 and 13, the laser-light detection sensors 105, 106 are disposed away from each other by a distance 115 which is greater than the entire width of an image forming region on the photosensitive drum 107. The plural pieces of laser light LB1 to LB4 are scanned along loci 111 which are shifted from one another in the sub-scanning direction.
In the measurement of a scanning time required for each of the pieces of laser light LB1 to LB4 to pass through between the two sensors 105, 106, time points where each laser light passes through a slit 113 formed in the sensor 105 and where it passes through a slit 114 formed in the sensor 106 are measured.
In a state that the laser-light detection sensor 106 is mounted inclined as shown by a dotted line 116 in FIG. 13, time points where respective pieces of laser light pass through the slit 114 vary according to sub-scanning direction positions in the slit passed by the respective pieces of laser light. For example, if a distance between adjacent pieces of laser light in the sub-scanning direction is 200 μm and the sensor 106 is inclined by an angle of 2 degrees, a distance between the slits is deviated by about 5 μm between adjacent pieces of laser light, resulting in an error in measurement values of scanning times required for the respective pieces of laser light to pass through between the sensors. As a result, it becomes impossible to perform proper correction of the main scanning scale factor. To obviate this, an adjustment to make the slits parallel to each other is generally performed. An adjustment mechanism therefor is disclosed in Japanese Laid-open Patent Publication No. 08-132670.
However, the adjustment mechanism has a problem that much effort is required for adjustment work since this mechanism is configured to be operated by an operator to adjust the slits to be parallel to each other based on a state of an image transferred onto a sheet material.
It is also possible to measure scanning times required for two different laser light beams each to pass through between sensors and adjust slits to be parallel to each other by adjusting slit positions such that the measured scanning times become equal to each other. With this adjustment method, it becomes possible to detect and correct deterioration of the parallel degree between the slits, which is caused by secular change or the like.
In the case of multi-beam image formation, respective beams have wavelength differences therebetween and have different refractive angles. As a result, differences occurs in the main scanning scale factor between the respective beams. Accordingly, in a case that there are wavelength differences between the beams, a problem is posed that the main scanning scale factor cannot be properly corrected, even if the slit positions are adjusted such that scanning times become equal to each other between the respective beams.
As for a light emission unit having laser elements integrated thereon, temperatures of these laser elements become different from one another depending on a use state of the light emission unit. In that case, differences occur in temperature-dependent beam wavelengths, posing a problem that the main scanning scale factor cannot be properly corrected.
Further, there is a method in which scanning times measured for beams are electrically corrected, without performing a slit adjustment to make sensor slits parallel to each other. With such a method, it is possible to correct scanning times of two beams used for the scanning time measurement, but it is not possible to correct scanning times of the other beams. In a case that beam scanning positions are shifted in the sub-scanning direction due to temperature rise inside the apparatus, the scanning times cannot be corrected without identifying the beam scanning positions after being shifted, so that a problem is posed that the main scanning scale factor cannot be properly corrected.